This invention relates generally to apparatus and methods for polarizing light and particularly to fiber optic apparatus and methods for providing light of a predetermined polarization. Still more particularly, this invention relates to a polarizer system including a feedback loop which provides an error signal porportional to the intensity of a selected polarization mode propagating in the fiber and means for nulling the error signal.
It is well known that in many fiber optic systems, it may be desirable to have light of a known polarization state at selected points for input to components whose operation is polarization dependant to minimize errors. The state of polarization is particularly important in a device such as an optical fiber gyroscope. In a polarized optical fiber gyroscope system, drift errors due to polarization are determined by the quality of the polarizer and by the control of the state of polarization.
A linear polarization state is typically achieved with some type of linear polarizer such as the fiber optic polarizer described in U.S. Pat. No. 4,386,822 to Bergh. The polarization state input to the polarizer is arbitrary in general. The polarizer couples light of undesired polarizations out of the fiber and permits light having only a selected desired polarization to propagate through the fiber. If none of the incident light has the desired polarization, then the insertion loss is 100% and no signal passes through the polarizer.
An improved apparatus for producing light of a known polarization includes a polarization controller placed in the fiber between the light source and the polarizer with the polarization controller being adjusted to output light of the desired polarization into the polarizer. However, in a typical system, the polarization state input to the polarization controller varies due to the environmental sensitivities of the optical fiber. Variations in temperature and pressure, vibrations, and ageing of the materials may cause significant changes in the polarization output from the polarization controller to the polarizer. Therefore, in a system which includes a polarization controller fixed to vary the polarization of input light by a predetermined amount, the time varying polarization of the light input to the polarization controller causes signal fading.
Most fiber optic gyroscope systems thusfar demonstrated include a number of bulk optic devices. These bulk optic devices include mirrors, lenses, beam splitters, prisms, and Bragg cells, for example. Although each of the bulk optic devices needed to implement a fiber optic gyroscope system currently exists, the losses, light scatter and non-reciprocal optical properties which they introduce into the system seriously degrade the performance of the gyroscope system even when the bulk optic devices are of the finest possible optical quality. Experiments have shown that an all-optical fiber gyroscope system can achieve a noise level of about 0.001 deg/(hr) 1/2. The best bulk optics equivalent has a noise level which is many times poorer than that of the all-fiber system.
In addition to purely optical limitations, bulk optics elements introduce limitations on packaging size, severe design constraints in the ability to withstand the temperature range required by typical military specifications, and potential sources of vibration sensitivity. Bulk optics devices of the required quality are extremely expensive, and experience has shown that it is unlikely that significant cost reductions can be achieved in the production of such bulk optic devices.
The performance of a fiber optic rotation sensor depends critically upon the state of polarization in the fiber. The state of polarization in a fiber is very much dependent upon environment, but the state of polarization can be electronically controlled.
A fiber optic polarization controller is a 2-port device which transforms an arbitrary input state of polarization into a desired output state of polarization. This transformation is accomplished by placing two adjustable birefringent sections in the optical path. The application of anisotropic stresses to the fiber induces birefringence through the photoelastic effect. The photoelastic effect relates the change in the indices of refraction of the fiber to the applied stress. Anisotropic stresses may be applied to the fiber by squeezing the fiber or by bending the fiber around a circular form. Bending the fiber around a circular form has been used with great success in the laboratory by manual adjustment of the angular positions of the circular forms, but such polarization controllers have not been found to be suitable in the construction of small and simple servomechanism for controlling the state of polarization in an optical fiber.
Experiments have been done on polarization stabilization in single-mode optical fiber using two electromagnetic fiber squeezers. These experiments use bulk optic devices to examine the state of polarization of the light exiting the fiber and to provide a feedback signal to control the electromagnetic squeezers. The apparatus used in these experiments has the disadvantages of requiring access to the ends of the fiber, being excessively bulky and having a high insertion loss.
In order to achieve the benefits which may be derived from the all-fiber approach to optical gyroscopes, suitable optical components must be available in forms adaptable to fabrication on a single fiber in order to minimize the losses and back-scattering techniques associated with splices. The components must be relatively small, lightweight, easily packagable and capable of meeting stringent operational specifications to provide an optical gyroscope system suitable for military and civilian guidance applications.